1. Field of the Invention
The present invention relates to a ribbon feed mechanism for use in a printer.
2. Description of the Prior Art
In recent years, heat transfer color printers have been used as computer aided design terminals or in computer graphics or video tape recording.
FIG. 1 is a perspective view of a heat transfer color printer. The heat transfer color printer includes: a lower cabinet 1; a circuit section 2; a power source panel 3; a pinch roller 4; a capstan roller 5 with which the pinch roller 4 makes contact to drive it; and a platen roller 6. The platen roller, the pinch roller 4, and the capstan roller 5 together form a paper feed mechanism. The pinch roller 4 and the platen roller 6 are driven by a main motor 7. The printer also includes: a pinch roller lever 8 for contacting the pinch roller 4 with the capstan roller 5 and separating the pinch roller 4 from the capstan roller 5 in synchronization with the movement of a cam; a peel-off roller 9 for separating an ink film from a sheet of paper; a paper feed roller 10 for contacting the sheet of paper with the platen roller 6; a paper guide 11 for guiding the sheet of paper; a paper support 12 for housing a roll of paper; a cover frame 13 which is opened and closed by the engagement of a locking lever 14 with a locking lever shaft 15 and the disengagement of the locking lever from the locking level shaft, respectively, the cover frame 13 having a ribbon feed gear (not shown) for positioning the one end of a ribbon feed which is a roll of ink film as well as a ribbon feed presser 17 for pressing the other end of the ribbon feed; a head holder 18; a head 19 fixed to the head holder 18; a heat arm 20 which is moved in synchronization with the cam and to which the head holder is fixed; a cooling fan 21 for radiating the heat from the main motor 7 and the circuit section 2; an operation panel 22; a top cover 23; and an upper cabinet 24.
In such a heat transfer color printer, if a ribbon-shaped ink film wound in a roll is to be fed, it is required that a feed mechanism of the ink film incorporates a slipping mechanism from the viewpoint of the diameter of the roll or the feed speed thereof.
A known ribbon feed structure having a slipping function will be described below with reference to FIG. 2.
FIG. 2 is a schematic cross-sectional view of the ribbon feed mechanism of a thermal type printer. This thermal printer is of a heat transfer type which employs an ink film 32. After rolled out from a recording paper roll 29, a sheet of recording paper 30 is passed between a capstan roller 25 and a pinch roller 26. The sheet of recording paper 30 is pressed by these two rollers and is conveyed by the drive of the capstan roller 25 to a platen roller 27, at which it is brought into contact with the ink film 32. At this time, the sheet of recording paper 30 and the ink film 32 are pressed by a thermal head 28 and the platen roller 27.
The thermal head 28 serves to convert electrical signals to thermal signals, and these thermal signals are used to perform recording on a sheet of heat-sensitive paper (not shown). Alternatively, the thermal signals are used to heat transfer an ink of the ink film 32 onto the sheet of paper 30, so that characters or images are sequentially recorded on the sheet of recording paper 30 in accordance with the electrical signals. The sheet of recording paper 30 is then cut into predetermined lengths by a cutter 31, thereby completing recording. During this process, the ink film 32 is rolled out from an ink film roll 33 and is conveyed by a supply feed mechanism 34 and winding feed mechanism 35. The supply feed mechanism 34 and the winding feed mechanism 35 of the ink film in general are provided with a slipping mechanism because the sheet of recording paper 30 and the ink film 32 are often conveyed by the same motor (not shown) and because the diameter of the ink film roller 33 varies in accordance with the quantity of ink film which has been fed.
FIG. 3 shows an example of the slipping mechanism. A friction plate 43 is fixed to a ribbon feed component 40. A ribbon feed component 41 is mounted on the friction plate 43 in such a manner that it makes contact with the friction plate 43. A ribbon feed component 42 is placed on the ribbon feed component 41 through the intermediary of a spring 44 in such a manner that the ribbon feed component 42 and the ribbon feed component 41 are fixed to each other in the rotational direction, and is fixed to a shaft 46 through a C ring 45. More specifically, the ribbon feed component 40 and the ribbon feed components 41 and 42 are fixed to the shaft 46 in that order with the friction plate 43 being interposed between the ribbon feed component 40 and the ribbon feed component 41. A sliding friction generated between the ribbon feed components 41 and the friction plate is determined by the force imparted by the spring 44. In this case, a paper reel 47 mounted on the ribbon feed component 42 is driven by the ribbon feed component 40 which serves as a driving source. An ink film is wound around the paper reel 47.
In this known structure, a torque that rotates the paper reel 47 is determined by a state of the contact surfaces of the friction plate 43 and the ribbon feed component 41 and the force of the spring 44. In the friction generated at this time, however, there is a great difference between a static friction generated before the slide is started and a dynamic friction generated after the slide has been started. In consequence, the rotational torque is not made constant and is therefore unstable.